Neodymium: yttrium aluminum garnet (Nd:YAG) crystals longitudinally pumped by semiconductor laser arrays have been established as a means of obtaining a moderate power continuous-wave (cw) laser emission at 1.06 .mu.m. Such a laser excitation scheme is highly efficient, compact, long-lived, and frequency-stable. D. L. Sipes, "Highly efficient neodymium:yttrium aluminum garnet laser end pumped by a semiconductor laser array," Appl. Phys. Lett. V. 47, 74 (1985), and B. Zhou, T. J. Kane, G. J. Dixon and R. L. Byer, "Efficient, frequency-stable laser-diode-pumped Nd:YAG laser," Opt. Lett. V.10, 62 (1985). See also D. L. Sipes, "Highly Efficient Nd:YAG Lasers for Free-Space Optical Communications," TDA Progress Report 42-80, Jet Propulsion Laboratory, Pasadena, California, pp 31-39, Oct.-Dec. 1984.
There are potential advantages in the use of Nd:YLF as opposed to Nd:YAG, such as longer fluorescent lifetime and inherently polarized oscillation. T. J. Fan, G. J. Dixon and R. L. Byer, Opt. Lett. V.4, 204 (1986). A Holmium:Yttrium Aluminum Garnet (Ho:YAG) laser optically pumped by a diode-laser array was later reported by R. Allen, L. Esterowitz, L. Goldberg, and J. F. Fowler, Electronics Lett. "Diode-pumped 2 .mu.m holmium laser," V.22, 947 (1986).
Holmium (Ho) solid-state lasers have been known for at least two decades, but have always had to be operated at very low temperatures (typically 77.degree. K.) or with very high input powers to overcome the lasing threshold. E. P. Chicklis, C. S. Naiman, R. C. Folweiler and J. C. Doherty, "Stimulated Emission in Multiply Doped Ho.sup.3+ :YLF and YAG--A Comparison," IEEE J. Quantum Electron., Vol. QE-8, No. 2, 225 (1972 ) The lifetime of the upper Ho laser level (.sup.5 I.sub.7) is approximately 12 ms. This value is about 50 times that of the corresponding metastable state of Nd:YAG (230 .mu.s) which indicates a high storage capacity for the YLF material.
What has been sought since the early part of the previous decade is a laser that is power-efficient, compact, has a low lasing threshold, and operates at room temperature. The early work in quest of a solution to this need began with Nd lasers. R. B. Chesler and D. A. Draegert, "Miniature diode-pumped Nd:YAlG lasers," Appl. Phys. Lett., Vol. 23, No. 5, 235 (1973); M. Saruwatari, T. Kimura and K. Otsuka, "Miniaturized cw LiNdP.sub.4 O.sub.12 laser pumped with a semiconductor laser," Appl. Phys. Lett., Vol. 29, No. 5, 291 (1976); K. Washio, K. Iwamoto, K. Inoue, I. Hino, S. Matsumoto and F. Saito, "Room-temperature cw operation of an efficient miniaturized Nd:YAG laser end-pumped by a superluminescent diode," Appl. Phys. Lett., Vol. 29, No. 11, 720 (1976). This search had not been extended to other activators besides Nd, and particularly not to Ho, until the present invention.
Ho solid-state lasers have been known for at least as long as this search has taken place for a miniaturized laser end-pumped by a semiconductor laser. See the 1972 IEEE paper by Chicklis, et al., in the Journal of Quantum Electronics cited above which compared Ho:YLF and Ho:YAG lasers sensitized by Er and Tm for high pumping efficiencies. But the comparisons were conducted with a xenon flashlamp for pumping at room temperature which necessarily means room-temperature operation at high input, broad spectrum input power. The conclusion reached was that YLF was the better host for Er,Tm,Ho because of its very high slope efficiency and low threshold.
It was not until 1986 that efficient Ho lasing was reported at room temperature. E. W. Duczynski, G. Huber, V. C. Ostroumov and I. A. Shcherbakov, "cw double cross pumping of the 5/7-5/8 laser transition in Ho.sup.3+ -doped garnets," Appl. Phys. Lett. 48(23) 1562 (1986). However, there Cr,Tm,Ho:YSGG and Cr,Tm,Ho:YSAG crystals were pumped with a Krypton laser with a suggestion that they be flashlamp pumped for efficient operation near 2 .mu.m wavelength. That does not satisfy the need for a power-efficient, compact solid-state laser with a low lasing threshold for operation at room temperature.